342 tague et al. 1998; Montague et al. 2000; Montague and Kjelgren 2004; Bush et al. 2008). Previous research indicates stomatal closure in response to increasing VPD is a mechanism to regulate plant water status and avoid damaging effects of water deficit stress, such as excessive cavitation (Bush et al. 2008). However, maple trees in this study did not exhibit such behavior. Increased gs in response to increasing VPD (gs therefore not controlled by VPD) has been reported for a limited number of tree species (Montague et al. 2000; Bush 2008), and because A is directly influenced by gs (Niu et al. 2008), this response is a possible adaptation to maximize gas exchange in environments where soil water is non-limiting (Montague 2000; Bush 2008). Differences among irrigation treatments were not found for trunk cross-sectional area increase (Figure 4A). However, across all irrigation regimes, shantung maple had 57% more trunk cross-sectional area increase when compared to ‘Autumn Blaze’ maple. Total tree leaf area was greatest for trees receiving the greatest amount of irrigation volume, and ‘Autumn Blaze’ maple trees had 48% more total leaf area when compared to shantung maple trees (Figure 4B). Shoot elongation data had an irrigation × species interaction (Figure 4C). Therefore, shoot elongation irrigation × species means are presented. For shantung maple, trees receiving the intermediate irrigation volume had the greatest shoot elonga- tion. However, ‘Autumn Blaze’ trees receiving the intermediate and high-irrigation regime had greater shoot elongation when compared to low-irrigation trees. Reduced apical growth is a common response for plants exposed to water deficit stress (Kramer and Boyer 1995), and has been documented in maple (Abrams and Kubiske 1990; St. Hilaire and Graves 2001). Although few studies have investi- gated gas exchange of field-grown (or urban-grown) trees in high-VPD, urban-like climates (Cregg and Dix 2001; Bush et al. 2008), previous reports on api- cal growth of field-grown trees subjected to reduced irrigation in semi-arid, high-VPD climates is sparse. In a study closely associated with this research proj- ect, in a high-VPD, semi-arid climate, Fox and Mon- tague (2009) subjected numerous tree species (Acer, Cercis, Fraxinus, Prunus, and Quercus) to variable, ETo-based irrigation rates (33%, 66%, and 100% ETo). They determined total tree leaf area and shoot elongation for field-grown trees were not always cor- ©2015 International Society of Arboriculture Montague and Bates: Response of Maple to Reduced Irrigation related with trees receiving the greatest amount of irrigation volume. Fox et al. (2014) reported compa- rable results for several redbud varieties. All reduced irrigation trees in research reported here were sub- jected to at least minor water deficit stress (Figure 2). Overall, apical growth for maple trees likely was reduced due to reallocation of assimilates. One of the mechanisms trees use to acclimate to mild soil drying is to shiſt allocation of carbohydrates from apical to root growth. Thus increasing root surface area by decreasing the apical growth fraction, but increasing the root fraction of total plant biomass (Khalil and Grace 1992; Kramer and Boyer 1995). Shantung maple is native to semi-arid regions of northern China (Pair 1987), and is thought to be Figure 4. Effect of irrigation volume (low = 33%, intermediate = 66%, and high = 100% of reference evapotranspiration) on A) trunk cross-sectional area increase, B) total tree leaf area, and C) shoot elongation for field-grown shantung maple (Acer truncatum), and Autumn Blaze maple (A. × freemanii ‘Autumn Blaze’) trees. Different letters indicate effect of irri- gation volume on trunk cross-sectional area, leaf area, and shoot elongation (Fisher’s least significance difference pro- cedure, P ≤ 0.05).
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